Add pluck keyof type for prop #1530

[FrankiePo]

Why don't we use typescript's keyof operator instead of just string in pluck operator? I'm saying we can convert pluck types from

pluck<T, R>(props: string[]): R {}

To

pluck<T, K extends keyof T>(props: K[]): T[K] {}

This stuff gives us an error on compilation phase for the following code

Rx.Observable.of({
    a: 1,
    b: 2,
}).pluck('c')

[msftclas]

All CLA requirements met.

[FrankiePo]

see also advanced-types